User specified gift certificate price field

I am trying to setup a gift certificate.

I would like to have a gift certificate where the customer can enter the certificate amount instead of pre priced amounts. Also, how is the remainder of a gift certificate is handled when a purchased amount is below the certificate amount?

I had not thought of using the donation plugin for dynamic prices, thanks. But I don't think I understand the "price link to the bought product" part. Is the the price linking going to cover the following:

A gift Certificate was purchased for 50 but the customer comes into the store and only spends 25 of the fifty, does your system keep 25 on the gift Certificate for the next time the customer comes in to spend the last 25?