Is it okay to ask to "waive" the stipend when offered a fellowship for PhD program? John just got accepted to a Ph.D. program in Computer Science. He also received a fellowship in the form of TA-RA-ships which comes with a stipend of $34,000/year for 4 years.  

However, John is currently receiving Disability Supplemental Security Income (SSI) from the U.S government which prohibits him from having any income.

He is only interested to have his tuition covered without receiving extra money.

1) Is it okay for John to ask the university to forfeit the stipend? (or for any other reasons than the above).

2) Can John ask rather to receive the stipend, to have the amount apply to his tuition after the 4th year?

John appreciates your thoughts on this... Thanks :)
You can certainly ask for the stipend to be withdrawn. Most universities are happy to spend less money. They may not be happy if this also means that you are not doing the TA/RA work, depending on whether they have enough TAs/RAs. 

You can also ask for the stipend to apply to tuition - no harm in asking. Less likely that this will be granted than just stipend withdrawal. 

Volunteer work is usually accepted, so if you want to do the TA/RA work you may be able to, even without the stipend. The legal status of such work is the question, which you should ask a lawyer about.