Medical Expenditure

1. As per explanation 2B of sec 80D, it reads as, 
(i)  any mode, including cash, in respect of any sum paid on account of preventive health check-up;
(ii)  any mode other than cash in all other cases not falling under clause (i).
2. From the above reading, it is clear that cash can be paid only for a preventive checkup with up to Rs. 5,000 and for claiming medical expenses (max of Rs. 50,000) u/s 80D(2)(c) it should be any mode other than cash. 
3. In your case, if the medical expenditures are paid in cash it cannot be claimed as deductions u/s 80D. 
Please correct me if the above solution has an alternative view. 

Can't do much. Hospitals generally accept payment other than through cash. Only small clinics will refuse to accept cash.