'The lawyer works at the bank' was not accepted. Does this mean that dla implies that the lawyer is not employed at the bank, but has simply been engaged by it?
dla banku = the lawyer is engaged or the lawyer is employed so he works in favour of the bank
w banku = at the bank (the lawyer is employed) OR in the bank (the lawyer works in building of the bank)
When it is in Genitive, or Locative. Here it's Genitive, as needed by "dla".