How can I effectively enter codeless paper gift certificates in the retail system?

[Blissful7]

Hi! I have taken over a retail business and the previous owner had gift certificates with no code on them and we agreed to honor them. What is the best way to enter them in the system if they don’t use the exact amount? For the exact amount gift certificates I’ve been doing a order discount example taking $50 off the order. I found that you can only enter an even amount not some number as in $45.78 in the $ discount section.  
hope someone can help out!

thanks!