не заключили бы контракт если бы знали, что положение не будет иметь силы

[real_patsan]

would not have concluded, или в контрактах в другом времени лучше написать?

[Little Mo]

we wouldn't have concluded the contract should we have known that this provision will be deemed invalid

кажись, так

[stray]

if we had known ... we would not have ...

я бы так - чисто имхо

и я бы не concluded а я бы signed или что-нить бы еще поискала... (хотя смарю - можно concluded...

[d.]

to LM: would_not, no contractions in contracts)
+ not "will be deemed", "would be deemed" - Future in the Past

[Tollmuch]

had [ ] known that... [ ] wouldn't have...

[AnnaB]

To Little Mo: А почему во второй части have known?
Должно быть had known. И тогда, соответственно, по правилу согласования времен this provision would be deemed

Это если смотреть только с грамматической точки зрения.

[d.]

We would not have signed the contract if we had known that Clause Z would not be valid/would become invalid/was invalid
в зависимости от того, было ли положение уже тогда недействительным, или стало потом
и никаких сокращений, это официальный стиль!

[Little Mo]

d: ta very much
AnnaB: снимаю свой вариант

[V]

имхо лучче

we would not have entered into this contract if we had known that... would be...