Mr Collins' exact words are "one thousand pounds in the four per cents., which will not be yours till after your mother's decease, is all that you may ever be entitled to". The "may ever" suggests that the distribution of the small Bennet fortune is not decided and Mr Collins knows this. Of course he may be taking into account the possibility that Mrs Bennet survives Elizabeth, which was entirely possible in an age when many young women died in childbirth.