Okay, I'm sufficiently confused. If I read the preceding three pages of posts correctly, and ignore the opinions from all sides, here's what I boil down:
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Fact #1: You bought a set of sights for Gun A from MidwayUSA.
Fact #2: You received the sights and subsequently installed them on Gun A, and threw away the package.
Fact #3: After using the sights with the gun, you decided to get rid of the gun.
Fact #4: You removed the sights from Gun A, replaced the original sights on Gun A, and sold Gun A.
Fact #5: You contacted Midway USA to ask for a refund of the purchase price for the sights that you previously installed and used on Gun A, and for those funds to be applied toward the purchase of similar (identical?) sights to be installed on Gun B.
Fact #6: You at no time misrepresented or neglected to inform Midway USA of the condition of the sights from Gun A that you wanted to return.
Fact #7: Midway USA agreed to accept the terms of an exchange based on the accurate information you provided them from Facts #6 and 7.
Fact #8: After the agreement had been made, they solicited you to purchase the sights for Gun B in advance, and that a credit would be provided upon receipt of the return.
Fact #9: Upon receipt of the returned sights, they contacted you to tell you that the previous agreement was null and void, and that in actuality they were not willing to accept the return.
Fact #10: After much wrangling by you, the manufacturer and Midway USA, there appears to be a potential solution to the problem.
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So, IF all those are actually facts, then I'd have to say the following:
1. I agree that Midway USA did not keep their word to accept the return, and I would be as upset as you were.
2. Knowing what you know now, in the future, you should request a written agreement before sending back the product that had been previously used (knowing that tool marks are a fairly universal no-no in the gun world).
3. Even if Midway USA had gone back on its agreement, the fact that they're willing to try to find a reasonable solution to the whole thing says to me that they're probably worth dealing with still.
4. Good luck. I've had nothing but positive experiences with the folks there, and have had to return things from time to time. As you've confirmed more than once, they are always courteous and professional, even if they don't say what you'd like to hear.
5. Welcome to THR.