I don't think Farley rejecting the Sixers, if it happens, will cause too much trouble signing a replacement. Unlike Australian Free Agents, who all get snapped up in the first couple of months after the season, plenty of US players are going to be available in August/September. In all probability most import candidates would hold off signing anywhere until just prior to the NBA season because they are all still hoping to land a roster spot there.
Waiting a couple of extra weeks in May is not going to hold things up to much, I don't think.