Centre Williams will be available for New Zealand's Rugby Championship opener against Australia on August 19.
He is currently banned after being sent off during the All Blacks' second Test defeat against the British and Irish Lions last month.
World Rugby revealed "surprise" at the decision of an independent appeal panel this week to deem New Zealand's August 11 encounter against Counties Manukau and Taranaki - with the All Blacks playing 40 minutes against each side - as meeting the regulations constituting a match.
They responded to the ruling by issuing a statement insisting it will respect Williams' right to face Australia, but that it will seek clarity on just what represents a match among rugby's complex rules and regulations.
The August 11 game will now count in terms of Williams' suspension - the disciplinary committee that suspended him last month had discounted it - meaning he will be available for the Wallabies clash in Sydney.
Foster, meanwhile, defended the so-called 'game of three halves' status as a match, which also sees Counties and Taranaki playing 40 minutes against each other.
"If people turn up to watch this game, does it look like a match? Of course it does," Foster told New Zealand's Radio Sport, with his comments being reported by www.allblacks.com.
"Is it meaningful? Yes it is. It's not a training run, and you think about the Counties and Taranaki players, they are going to be going for it. It's a great opportunity for them, and it's 80 minutes played under the normal laws of the game.
"From a common-sense perspective it is a game we play 80 minutes under the normal laws of rugby.
"We said that we've done this three or four times, so it is not like a one-off. It's not something that has been hastily organised. The reason we play two teams is because it actually makes the game harder for us.
"If someone gets sent off in this game then they have to go through the same process Sonny had to go through.
"For all intents and purposes, this is a match, and we just wanted the chance to express that and, fortunately for us, we felt that common-sense has prevailed."
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