To start, if the equity amount is positive the Variance seller is the payer.
Then the equity amount = variance amount * min(FRV, Variance Cap Amount- Variance Strike price)
The default under the ISDA is for the Variance cap amount to be 2.5* Variance Strike Price. These are set when the transaction is entered into. FRV stands for Final Realized Volatility and in the case of the cancellation of a contract, it is calculated as follows
\( FRV= 100*\sqrt{\frac{252*\sum^{t_c}_{t=1}\large(\ln(\frac{P_t}{P_{t-1}}\right)^2+ (T-t_c)v_r^2}{T}}\)
I'm using a slightly different notation than the ISDA.
\(T\) is the number of trading days (Observation Days in ISDA parlance) the swap is in effect (ie. for a 1 year swap, it would be 252)
\(t_c\) is the number of trading days that have occurred since the contract was entered into until the contract cancellation/ termination
\(v_r\) is the mid market volatility of the underlying for the remaining term of the swap at the time of cancellation. (1 year left then you would look at 1 year options)
Note that if you let \(t_c=T\) you get the FRV for a swap that is not prematurely canceled.
A a lot of ISDA documentation can be found online through clever use of google. Also the supplement on which I'm getting this info clearly defines what happens at the end of the contract and is thus much simpler calculation than would be required for the mark to market. Disclaimer: I'm not an expert in this, I just decided to read ISDA legal framework (which is riveting stuff after reading the Basel II and Basel III accords) after reading this article:
Feldstein who Speared Whale Ready to Unwind Derivatives