Tactically it's a very bad idea. As mentioned in other answers, it only make the Siege harder. However, under certain strategic and political situation, this can be a plausible action.
Let's first analyze the possible benefit of doing so for the siege side (considering the side under siege would accept the offer):
- Possibly keep the army under siege where they are, so there won't be any unexpected activity (like trying to break the siege out of desperation)
- Reducing the casualty in the castle, this can work in all different ways that I will elaborate below
- Getting the gold right away
So yes, as long as breaking into the fortress isn't your primary goal, we can imagine some scenarios that make this plausible:
A token force trying to keep a fortress and its army in a strategically non-important place contained without paying too much casualty.
Bonus if the local vassal also don't want to get involved in the larger scaled war but still have to show some level of resistance, then a hidden deal can be signed, the siege side maintain the siege, while also keep the fortress army well fed so they won't do anything unexpected.
A war that near ending, yet there might be a key general under siege in a fortress, which can be a huge interruption for any future war effort for the attacking side. Thus attacker can lure the defender with a offer: provide the fortress with needed food for money.
Trading with enemy during war time is a treasonous act, and can provide some serious political tempest inside the defenders, which might be even more efficient than just killing the general. A general killed in war is a hero; a general getting executed after war for politically reason though, with the generals supporters, can easily split or even destroy a country from inside.
Or of course, you can always keep the truth as a timed bomb and keep the general's hand tied, or even better, create a puppet in the enemy military command system.
Again a war is ending, and the fortress under siege is part of the truce deal, yes, the attacker side will be their new lord. Thus attacker may choose a softer act to avoid frictions in future rules, after all, it's very likely that the soldiers inside the fortress are just drafted from villages around it.
The Great Khan is dead, and we are all going back home for the new Khan's election in two weeks. It's very unlikely we can force ourselves into the fortress in two weeks, not mentioning the casualty we have to take (soldiers are important resources considering the upcoming battle for the new Khan's throne). How about let's make a deal with the guys in the fortress for some gold that may be useful in future.