Flooring Tile - adding wet grout to dry

We are tiling a bathtub and ran out of grout about halfway down a wall. We were unable to get any more grout for several days. I was told if we try to finish it that where the wet grount joins the dry grout we will get a crack. My husband insists that I wasn't told that and that we won't have a problem. Can anyone advise me on what to do? We are going to be selling our house and I do NOT want to sell a house with something that I KNOW is going to be a problem. Also, we do have some hairline cracks in it already and wondered if there is a way to repair them? Thanks so much. Jeanette

You don't have any choice now. Just do it. It'll probably be fine.

We just got done putting a total of over 600 sq ft of tile. The grouting was done at the rate of, at the most, 30 sq ft at a time by my wife. Absolutely, no problem.
Just a heads-up; the color of the grout will look different for some number of days but assuming there is no color difference in the grout bought at different times, it all will look just fine. Additionally, as John said, you dont have a choice!