OnePlus India — one plus 3t model no a3003 purchased 4 months back not working service provider asked 20000 rupees to repair

Purchased oneplus 3t model no a3003
Imei no [protected], invoice dated 16/03/2017. On 20.07.17 phone got switched off while talking with my friend. Taken to kurla service centre. They gave job report stating mother board has been damaged due to moisture.. Have to spent 20k for the same.. Unable to understand how the motherboard got moisturised when phone was used indoors only.. Not happy with the services.. Within span of 4months phone is damaged.. And company is asking 20000 to replace motherboard.. Look into the matter please.