How about making the border thinner at that point. I've seen lots of quilts where the pieces in the quilt "spilled" over into the border. Open a few stitches at top and bottom of that block. Put the border on that side as on the other side, but only up to the too-big block. Then add border fabric to that block. Sew the border seams (that now will include a little of the block, also) straighten out the edge and everything will be square.