True, but you can oftentimes shave off or add a few credits and still get the deal approved and fiddling with the current controls make that tedious.
On the other hand, how about just eliminating the input for credits and let the AI set the amount that is absolutely required for the deal to be accepted. This would make the process of including credits as part of a deal so much easier. There really is no need for the player to input values over and over to find the 'sweet spot', just have the AI calculate the credits needed and throw it into the pot. If the player adds or subtracts other items from the deal then the AI can update the credits needed accordingly.