That, or he just likes throwing away his gil.

I agree with Phogg. Either shout/have an LSmate make it/suck it up and not do that day's item. Yes, it's nice that it's only one or two synths for 5k GP, but if it doesn't happen, then don't cry over spilled milk. If anything, reward him after he makes you a dozen or so, and keep the rest on a mule for when this happens again.