Can an employer refuse your resignation against the agreement ?

Hello Sir, My existing employer has sent me abroad for 3 weeks for one particular project related purpose. However, they rook sign on one term & condition, saying that "You are being sent on the overseas trip for a specific purpose on the condition that you shall continue in the service of the company for a period of 1 year from the return of the date.This is to ensure that the organisation fully utilizes the benefit of your intended overseas trip". The employer is trying to take the advantage of this policy to retain the employee. However, the project intent is completed on the trip, but employer is putting condition to pay the trip amount,in case you put resignation before 1 year of return. Is it possible to challenge this condition of employer?