I'm clearly not a man, but I agree with what TKGS said.  

I do think that the specific details of your situation matter.  Perhaps you can shed more light on them?

I agree also.  So, for instance, if I offer to lend you $1,000 and then find out you plan to use it to pay for an abortion ... all bets are off.  Giving your "word" is not absolutely binding.

Are you in Minnesota? In another thread, it was posted at the SSPX is engaging in an ecuмenical activity with the FSSP.

https://www.cathinfo.com/sspx-resistance-news/sspx-kicking-off-holy-week-with-tradcuмenism/

I bet I know who is asking for advice...

You're off the hook, buddy. When I read your OP, I thought you wanted to back out because of some personal grievance. In that case I would lean towards having to keep your word, other things being equal.
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But your objection is based on serious moral issues and possible condonement or assistance in evil. You're certainly exempt from fulfilling this.
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That being said, would an honorable man attend a church at which he cannot in good conscience donate a set of vestments? That might be something you want to consider. I'm leaning towards "No".

He said they were salmon colored, so he didn't wear them.

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That's funny. My old priest in the SSPX also refused to wear the Rose vestments. The reason he gave was because they were the wrong color. He said they were pink and not rose. To him they were supposed to be "salmon colored."

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I hate to say something bad about the clergy, but to the OP, you might as well save your money, because this man will not wear anything you buy.
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I have heard numerous priests agonize and quibble about rose vestments, agonizing over the precise shade they are supposed to be, and how they are not supposed to be pink, but salmon-colored or whatever else. (By the way, the rubrics use the word "rose-colored", and say nothing about salmons.) Many priests refused to wear rose vestments because they were supposed just not quite the right hue or shade.
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Finally it dawned on me what was really going on. They just have the normal male repugnance against wearing anything that looks pink, and are trying to hide it by quibbling about roses and salmon and similar things. He's basically saying, "Eeeww, pink! I'm not wearing that!" While it's perfectly normal and healthy for a man to refuse to wear pink in general, to allow such an attitude to trump a liturgical directive is wrong and childish. But priests are human beings like anyone else, and unfortunately some of them just can't bring themselves to wear rose-colored vestments.
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So if you buy another set that is supposedly the right hue, he will probably not wear them anyway, or if he wears it once or twice, he will eventually be replaced by another priest who will say they are either "too pink" or "too salmon", whichever seems to fit better.

Calling anything w/ the FSSP ecuмenical is harsh. They're Catholics too.

Well this forum is dominated by members who do not believe the FSSP has valid holy orders.