While buffers resist pH change when acids or bases are added to them, they do change pH to some degree until the buffer is exhausted. Working a problem of this type requires two concepts: first, you need to do the stoichiometry to find out how much of the buffer was used up by the acid or base. After this, you need to use the equilibrium constant expression to compute the concentration of H⁺, which gives you the pH. The basic procedure is as follows:

1. Identify all the species. Write the reaction between the buffer and the added acid or base.
2. Compute the number of moles of acid or base added. Do stoichiometry to figure out how much buffer reacts with the added acid or base. If all the buffer reacts, compute the pH based on the amount of added acid or base left after the reaction. If not, go to the next step.
3. Write the equilibrium expression for the buffer
4. Use the usual method of computing the equilibrium concentrations of a solution to compute the concentration of H⁺
5. Compute the pH from the [H⁺]

Information provided by: http://learn.chem.vt.edu