If the nominal money supply and prices increase by the same percentage, then the demand for and supply of real money balances stays the same. The change in demand is not proportional to the change in real income. A one-time increase in prices increases prices only one time.
Inflation is a sustained overall price increase as measured by a price index.
A one-time increase in prices does not affect the demand for real money balances. Expected inflation reduces the demand for real balances. Firms want to hold real money balances to consummate transactions. In deciding the optimum amount of money balances to hold, a firm will want to minimize money holdings, subject to the constraint of having enough money on hand or access to it when its needed. If interest rates on time deposits rise relative to interest rates on checkable deposits, the demand for real balances will fall.