It's a careful game of politely telling the collector they are full of BS or wrong. Collectors know about substitution so they might be trying to scare you, other times they might be just trying to get enough urine from you because they ask for just a little more than what is needed to make sure they get it.

Read in this thread.

http://boards.cannabis.com/drug-test...oz-enough.html

Technically, if you gave them 60 and they tried to say you failed to give a sample, they would have to write in the comment field why and send in the sample anyway and hopefully the MRO would see it and know that 60 was given and should have been accepted.