Using a Return of Capital (RoC) transaction to reduce the shares basis works fine with the Advanced Portfolio Report (APR), but not with the Trial Balance (TB). This makes using the TB to validate the validate capital gain/loss value of other sales more complicated than it should be, as you need to allow for incorrectly handled RoC.

I documented RoC, after seeing http://lists.gnucash.org/pipermail/gnucash-user/2005-January/012667.html, in https://www.gnucash.org/docs/v2.6/C/gnucash-guide/invest-retofcap.html for https://bugzilla.gnome.org/show_bug.cgi?id=765131 before I realized it caused the TB to go out of balance.

How to duplicate the problem:

With Trading Accounts not on, and only 1 currency used, GnuCash 2.6.16, Windows 10.

Tx 1 01/07/2016 Opening Balance
$1000 DR Assets:Current Assets:Bank1
$1000 CR Equity:Opening Balances

Tx 2 01/08/2016 Tfr Bank to Brokerage
$500 DR Assets:Investments:Brokerage1
$500 CR Assets:Current Assets:Bank1

Tx 3 02/08/2016 Buy 500 Stock1 for $1 Ea
$500 DR Assets:Investments:Brokerage1:Stock1 (Shares 500,  Price $1)
$500 CR Assets:Investments:Brokerage1

Tx 4 03/08/2016 Return of Capital
$100 DR Assets:Current Assets:Bank1
$100 CR Assets:Investments:Brokerage1:Stock1 (Shares 0,  Price 0)

The Trial Balance as at 3/8/2016 (Commodities Price Source: Nearest in Time) shows:

$600 DR Assets:Current Assets:Bank1
$500 DR Assets:Investments:Brokerage1:Stock1      Expected $400
$1000 CR Equity:Opening Balances

DR Total $1100
CR Total $1000

I have not been able to find a way of recording a return of capital that works with both the TB and the APR.

See discussion http://gnucash.1415818.n4.nabble.com/Trial-Balance-does-not-include-amount-of-Return-of-Capital-Investment-splits-td4691653.html#a4691657