How can apply vender credit against bill..when create journal entry

How can apply vender credit against bill..when create journal entry

Dear ZOHO Team, 

How can apply vender credits against the bill.. When create a journal entry 

I Have tried to Create thought vender create Module.. but here the problem is 
associate bill compulsory 

Actually, this credit is not any bill against that is our commission income 

How can manage it? 

Kindly advice