Discount as Part of Tithe (Ma'aser)

Hello,My wife is a speech therapist. If she gives a discount (from her regular price) for treatment to a family of an avrech, can the difference be considered as part of the tithe (Ma'aser)? Thank you very much

Shalom Rav,

The discount can be calculated from tithe (Ma'aser) money. Provided that the agreement with the client is initially on the higher price, and then the reduction will then be considered as a discount. 

Because if you initially agree on the lower price, he will no longer be obligated to pay you more, and therefore the reduced price will not be considered as having given him charity, since he wasn't obligated to pay more.