from a programmers view point..................
I would think it in poor taste to sell the exact same script, as far as the idea.. you have learned your lesson. Dont give programmers ideas you dont want them to use unless they sign an agreement not to use the idea "nda".
While i havent found myself in this exact scenario, sometimes people do ask for customization of scripts and i then use and resell those ideas, but they arent the main part of the script...
example. lets say i built a program called "notepad.exe" and you want the program but you would like to add a feature " it would be great to have such and such added " so i customize it for you , i dont see anything wrong with using that idea unless the person has asked me not to ( and people do ask these things from time to time )
Infact heres a real world situation that is quite similar to yours..
YOUTUBE. has its flash video player. they licensed the software from another company ( to do video to swf ), of course being a large company ( youtube ) they wanted certain customizations to the script, these same customizations are now available to anyone else who buys the same software.. now we dont know if it was youtube who thought up the ideas or the software writer but i imagine it was prob a bit of both.. kinda sucks for youtube that its unique features can be used by anyone else who buys the software right..
In your situation it is a "mod" not a complete program so while i do feel for you i cant really blame the programmer for seeing the potential in the idea and wanting to profit from it..
Personally if i was your programmer i would have said " hey this idea has potential" and split the profits.. the key thing most people miss is, anyone can program an idea, not everyone can think up ideas for a program
so theres value in the original person who thought up the idea.. not just the software. i would rather have %50 of 3 great ideas than 100% of one..