And if they argue with you, say: "Allah knows best what you do."
This is like the Ayah:
وَإِن كَذَّبُوكَ فَقُل لِّى عَمَلِى وَلَكُمْ عَمَلُكُمْ أَنتُمْ بَرِييُونَ مِمَّأ أَعْمَلُ وَأَنَاْ بَرِىءٌ مِّمَّا تَعْمَلُونَ
And if they belie you, say: "For me are my deeds and for you are your deeds! You are innocent of what I do, and I am innocent of what you do!" (10:41)
...
اللَّهُ أَعْلَمُ بِمَا تَعْمَلُونَ
Allah knows best of what you do.
This is a stern warning and definite threat, as in the Ayah:
هُوَ أَعْلَمُ بِمَا تُفِيضُونَ فِيهِ كَفَى بِهِ شَهِيداً بَيْنِى وَبَيْنَكُمْ
He knows best of what you say among yourselves concerning it! Sufficient is He as a witness between me and you! (46: 8)
Allah says here:
اللَّهُ يَحْكُمُ بَيْنَكُمْ يَوْمَ الْقِيَامَةِ فِيمَا كُنتُمْ فِيهِ تَخْتَلِفُونَ