I travelled on this about 2 years ago. I walked across the border, but caught the bus back. Interestingly, if I remember correctly, the Frankfurt day ticket was also valid on the 983 in Poland. I was expecting to have to pay a special fare.• Bus route 983 runs from Frankfurt (Germany) to Slubice (Poland) and is operated by the German based Stadtverkehrsgesellschaft Frankfurt (SVF).
My late afternoon journey back on an articulated bus was very busy, mostly with German shoppers, presumably with Poland having cheaper prices.
Like other places mentioned, Frankfurt/Słubice used to be one city, but the border cuts through on the River Oder.