While preparing amended return, I realized that I am now missing the receipt for one of my charitable donations. Should I remove the donation when I amend the return?

That is allowed if you believe you will not be able to support the deduction in an audit situation. 

We combined two houses and gave a house full of furniture and home goods to Purple Heart.  They picked up multiple truck loads and left two separate receipts.  I entered as two separate donations with different dates.  I can only find one of the receipts.   I do have a spreadsheet that I used to inventory and value all of the items.

Again your decision ... if you are audited can you support your position at that time.  Will the auditor believe your story.

The spreadsheet, together with the one receipt and a copy of this discussion, should be enough to convince an auditor that you made the donation. I would not remove it. Save everything until three years after you file the amended return.

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you would produce a receipt only if you are audited.