[This response should not be construed as a Halachic ruling on the question. Rather, it is an exploration of the topic to help our readers become more familiar with the issues involved.]

The Halachah would seem to be that a craftsman does not acquire the improvements of the utensil (see Rambam Hilchos Sechirus 10:4, Shulchan Aruch CM 306:2). However, the Shach (ibid.) Paskens that it is a Safek, as many Rishonim indeed Pasken he does acquire the improvements of the utensil. Due to the Shach, it would seem that either claim would be accepted by a Beis Din (using "Kim Li") in order for a person to not have to pay out money. [The reasoning behind the various opinions are based on a lot of information, and beyond the scope of this forum.]

[I hope this response has clarified the issue. For all practical questions on the issue, please consult your local Orthodox rabbi.]

All the best,

Yaakov Montrose