He lies before 'the lie' ...
The first official "lie" (i.e. the amount in his bank account) is not the first lie Mark (Gervais) tells in the movie.
When he is on the first date, Jennifer Garner's mum rings and is pretty disparaging about him. He then asks JG how her mum is - she's says fine, to which he replies "great".
I doubt he was pleased enough to find out the fine condition of JG's mum after her ribbing of him, hence twas a lie, albeit a smallish one.