The relation between RoContractsBonusesEntity & RoEarningsTypesEntity seems like 1:m relation (RoContractsBonusesEntity.EarningTypeId = RoEarningsTypesEntity.Id)
So there 2 cases here:
Case #1:
You want to display a ReadOnly datagrid with RoContractsBonusesEntity records and showing the Description field from the RoEarningsTypesEntity table
For this case it's best that you create a field mapped on related field from the designer, thus adding the extra Description field to the RoContractsBonusesEntity, and you can display it normally like anyother field
Case #2 (which seems to be your case):
You want a datagrid with RoContractsBonusesEntity records allowing the EarningTypeId to be changed according to selection from a combo box that hold Descriptions from the RoEarningsTypesEntity.
For that you should load your combo from a collection of RoEarningsTypesEntity, and you should handle decide whether to handle the selection changed at once or wait till the user saves all his changes (my favorite scenario), when you will map the selected value to the RoContractsBonusesEntity.EarningTypeId.