Linking an actual $Function to another table


Love Airtable! Ok this is what I want to do, hopefully you can help me figure it out asap!

I have a client table, a budget table, and a freelancer table. On the budget table i have for example $20,000 I want to bill the client. Is there a way to link the actual $20,000 cell to a client that is also a Function? Basically that way we can create invoices at the same time. It seems like you only let us link things together, but we aren’t able to link functions together!