If any employee member shall be disabled (from causes which would result in duty total disability) to the extent that he cannot perform in full the particular duties of his position but shall be able to perform other duties on his particular force, to which he may be assigned, then such fireman or policeman shall receive a monthly partial disability benefit of one-half of the difference between the rate of pay he was entitled to receive at the time his disability was caused and the rate of pay of the position to which he may be assigned.