Retirement tax questions

I want to withdraw $20,000 from previous employer retirement plan.  They are holding 20% for taxes. So I'm getting a check to put in bank for $16,000. Within 60 days I'm putting it somewhere else that I haven't decided yet.  So when I send the $16,000 to the new place is it a completed transfer or do I have to add another $4000 from somewhere else to not be taxable on everything?  If I can only send $16,000 because the old plan is holding my taxes, does it blow up the transfer?