Let \( C \) and \( P \) be the cost price of the computer system and the printer respectively.
\[
\text{Selling Price of Computer and Printer in Case I: } \text{SP} = 0.9C + 1.2P
\]
Since he did not lose anything:
\[
C + P = 0.9C + 1.2P \quad \Rightarrow \quad C = 2P
\]
Now, Case II:
\[
\text{Selling Price in Case II: } \text{SP} = 1.05C + 0.85P
\]
Given that there was a loss of Rs. 800:
\[
\text{Loss} = C + P - 1.05C - 0.85P = 800
\]
\[
\Rightarrow 800 = C + P - 1.05C - 0.85P = 0.15C + 0.15P
\]
Substitute \( C = 2P \) into the equation:
\[
800 = 0.15(2P) + 0.15P = 0.45P
\]
\[
P = \frac{800}{0.45} = 1777.77
\]
So, the cost price of the printer is Rs. 16,000.