Deleting cell values, but not forumlas?

I work for a bank and we use an excel spreadsheet that has an in depth payment calculator used for creating monthly payment arrangements or calculating settlements on bank loans. I would like to add the following feature and I'm not quite sure exactly how to do it. For example, imagine cell A1 will contain the full payoff of a loan. I want to be able to put a settlement offer in B1 and have C1 populate the percentage of the payoff that is. At the same time, I want to be able to delete the settlement offer in B1 and put a percentage in C1 and have it calculate that settlement offer in B1. The calculations/formulas to do these problems is simple, but what I need is to not lose the formulas in B1 and C1 if I were to go back and forth entering settlement offers vs settlement percentages.

Any suggestions?

Here is a macro-enabled solution

If you update either A1 or B1, then C1 will adjust automatically
If you update C1, then B1 will update automatically.

Macros must be enable for this to work!

If your settlements are always above $100, you could use this file. It's a bit redecorated, though. Type your percentage OR amount in B, and automatically get the other value in C.