arrear salary bill is possible in cosa for those who are drawing their salary through grant-in-aid bill(like staff of sponsored institution), because grant-in-aid salary bill has only one detailed head i.e.[01]. but in case of govt. employee you can try it by entering or adding the arrear pay,DA,HRA,MA etc. manually in cosa in a particular month of salary bill & a sanction order for that amount may be enclosed with the bill.
