So my old employer contacted me recently and said they had accidentally paid me for the month after I had left. I checked my accounts and this is true - they paid me for the full month rather than just the week they owed me.

However in the process of finding this I went through all of the payments they made to me and noticed they seem to have underpaid me for my first two months and week of work.

Overall it would still work out I owe them - but not the full month worth of money they are probably going to ask back. I'm just wondering where I legally stand on this, can I rightfully ask they take into account what they owe me and not take back the full month (minus 1 week) worth of money? Is this something I can negotiate with them or do I need to get a lawyer?

Thanks.