It has been said that if keeping fast in this month is difficult one should give one Dirham or one Mund(750 gms) Wheat or Barley as Sadqah. In one tradition one loaf of bread has been recommended. If one is unable to keep fast and is also poor enough that he cannot even give prescribed Sadqah then he should recite the following 100 times daily through the month of Rajab.
سُبْحَانَ الإلهِ الْجَلِيلِ! سُبْحَانَ مَنْ لا يَنْبَغِي التّسْبِيحُ إلاّ لَهُ! سُبْحَانَ الأعَزِّ الأكْرَمِ! سُبْحَانَ مَنْ لَبِسَ الْعِزَّ وَهُوَ لَهُ أَهْلٌ.
'Dua in lieu of fasting' is tagged in Rajab collection. See other dua's from Rajab