Legal Question in Intellectual Property in Texas

You don't have a provisional PATENT. There is no such thing! You probably mean you have a provisional patent APPLICATION.

To answer your question, perhaps they could. It depends on how significant and necessary the "tweak" is. However, if you get good solid patent legal representation, you should be able to fashion so called "dominant" claims that would give you the right to prevent the other company from selling the tweaked version without your permission. However, the tweaker may be in a position to apply for and obtain claims covering the tweak and the tweaked product and perhaps the method of tweaking and the method of using the tweak, and thus prevent you from making using or selling tweaked products.

If you had solid patent attorney help, you might have prevented this situation by appropriate contractual "grantback " provisions. You need to get that aspect covered right away or you could indeed lose your negotiating position, particularly if you are talking to a Japanese company or Chinese company, who are notorious for tweaking their way into dominant positions on products.

Bottom line, get expert patent attorney assistance pronto or you stand to get snookered.by tweakers.