how to fix bills in collection?!?

my husband has a hospital bill in collection. ive heard if you make payments on it that it will hurt your credit more. is it true? were trying to build up his credit. also i read that if you are making payments to write a letter to the Collection agency and have them write an agreement to remove it from your credit once its paid off. has anyone went through this and what did you do to fix it. please help! ive read stuff online but its so confusing to me.