In a historically accurate feudal society, this would not be the disruption that we think it is in modern day. There are many workarounds to this there were actually used at some point. Some still are in places today.
Bartering and Credit
Many feudal societies did not use cash for trade. They either traded goods directly or kept a ledger of what each customer owed. In a time where it was costly and rare for people to travel far, local small business owners established trust with customers and kept track of debts for long periods of time. When most people worked in agriculture, it was not unusual to wait until the annual harvest to pay off debts.
Bargaining and Haggling
Merchants were wealthy skilled salesmen who convinced customers to buy their goods at the highest price they were willing, they performed many of the roles a multimillion dollar advertising industry does today. In a society where haggling and bargaining is widespread, a ban on a number would be a trivial inconvenience.
“Those are $8 each”
“I’ll give you $14 for 2”
Split Bills or Tipping
It would also be fairly straight forward to split or combine bills to avoid explicitly charging 7 units of currency. Either pay for each item separately if a bill comes to a total with 7 or wait until bill doesn’t contain one to settle the tab. In a society with tipping, it would be fairly easy to round up the bill to avoid charging 7 units as well.
Using valuable goods as currency
It was also common for valuable items, rather than cash, to be used as currency. In the past, gold, silver, rice, and salt have all been used as currency. I’ll pay with this piece of gold that happens to weigh 7 grams but the shopkeeper will weight it discretely.
Using foreign currency
When a country becomes economically unstable, it is commonplace for foreign currencies to be used in place of or alongside the local currency. The US dollar, Euro, and Japanese Yen are often still used in Asian and African countries where hyperinflation is an issue. A dual currency system or black market for foreign currency would be simple to implement if it became impractical to use the local currency. The 2 currencies could also be used interchangeably to avoid using 7s.
“That’ll be ¥13,000”
“Here’s $100 change” (not ¥7000)
Lack of enforcement
A King or Emperor does not have as much power as many believe. Often they were only a figurehead. Even those with real power, required a committee or cabinet of advisors and the loyalty of local Lords to effectively rule. Before modern transport and communications infrastructure and technology, it would be unthinkable for one person to run an entire country, especially one composed of many diverse states. Even an Emperor could convince their own government to enact such a whimsical law, it would be difficult to enforce.
Many states, provinces, and prefectures had far more autonomy than today. This was necessary to prevent revolts in a large disparate Empire. Many Emperors struggled to even collect taxes from the outer regions of their dominions, for example the Tokugawa Shogunate of Japan and the British Colonies in America. This was not a trivial issue to solve. Feudal states often had their own currencies, units of measurement, dialects, and militias. Feudal Lords were in charge of collecting taxes from their domains and passing on a share to the Emperor. Currency was either issued by local banks or measured in local units of weight. It was not uncommon for Lords to be dishonest about how much tax revenue they collected to avoid paying more to the Emperor. (This was one of the issues with Imperial units, they differed between regions). When each region has their own language, it would be nearly impossible to ban a word either. They’ll just use a different one that the imperial authorities don’t know. It would be far to difficult to enforce to be worth the effort for many regional authorities.
If each state is following their own laws and the Emperor has little control over them, how would they even know if they regional Lords were complying with the law or enforcing it. What is likely to happen is that they Emperors decree would largely be ignored. Perhaps a few people will be made an example of but it won’t be widely enforced. For those in the provinces, it will be just another day of politics in the Capital: a moment of vanity of an Emperor with a short-lived reign to be quickly overturned by their successor or a flagrant excuse for the Emperor to purge any who oppose them.
It would be a few days of chaos in the capital before workarounds are in place with little to no impact of trade or the economy as a whole. Feudal societies were already a chaotic patchwork or regional systems and it was already difficult to trade between regions, even within a Empire (so exotic goods were very expensive). A minor inconvenience would not impact such an economic system that is already so complex and inefficient.