Yup, just had a play...

DECLARE @TestClient varchar(11);

DECLARE @SQLString nvarchar(500);

DECLARE @ParmDefinition nvarchar(500);

SET @TestClient = 'somevalue';

SET @SQLString = N'EXEC [database].dbo.Test @Client'

SET @ParmDefinition = N'@Client varchar(10)';

EXECUTE sp_executesql @SQLString, @ParmDefinition, @Client = @TestClient;