In that case, them sponsoring the visa for you isn't huge at all. It's costing them nothing, and it's either allowing you to stay in the country and work, OR, you don't need it because you will be going to school! 🙂
As an aside, do you happen to know how long it would take to process paperwork for a foreign student attending medical school? Depending on the answer, it might turn out that foreign students being called off a WL in July just isn't a thing, in which case you are worrying about nothing! If you have a med school acceptance now or in the next few weeks, no reason to create bad karma just to make a few bucks for a few months. You never know when you might cross paths in the future.