How to change a flat rate irregular payment to user determined?

Summary:

I'd be grateful if anyone could give me advice on how to achieve something the business have asked for. We currently have an irregular payment allowance that pays a flat rate. This seems to come from a Calculation Value Definition, which shows on the Element setup. The business now want this to be an open allowance, i.e. any pay value can be input, no longer a flat rate. It is possible to amend the existing setup to achieve this? If so, how would I go about changing the element so that it is no longer fed by the Calculation Value