tax amendment changing filing status: where to put the tax due my husband paid already to the IRS

We filed 2012 taxes with Married Filing Separately status; however, we want to change it to MFJ. I already have received the tax refund for my part of the taxes and my husband paid what he was due. Now, I cannot figure out where to put this amount that he has already paid to the IRS.

Thank you
    To answer the question directly, his payment will be figured into the amount on Line 15 of Form 1040x.

    To change your filing status, you need to file an amended return, Form 1040X, that details the changes in your tax return from married filing separately to married filing jointly.  This is somewhat tricky in this case. When you fill out Form 1040x, you choose one of you to represent the "primary taxpayer" that represents the starting point for the amended return.  The line items on that person's return becomes the "original amount" in Column A of Form 1040x.  The net change represents the change in the line item on Form 1040X that occurs because the second taxpayer is being added to the return.  For example, suppose the primary taxpayer's adjusted gross income is $50,000, and the second taxpayer's adjusted gross income is $40,000.  The Line 1, Form 1040x, column A is $50,000, column B is $40,000 and column C is $90,000.

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