Advice needed regarding otp

Recently somewhere in April we met a couple whos selling their house, after viewing we both agreed that,
1) me n my wife will buy the house at whatever the valuation price is, no cov involve
2) since they will only be submitting their valuation only the following week(since they will only meet their minimum occupation period the following week) we agreed on signing the otp first( both party and leaving the valuation,price n date out) and paid them a deposit+option fee of total $1000.
3)both party decided not to engage an agent as we both r short on cash.

In late april they called us up n they said they decided they wanted to do another valuation as their valuation only came up at $405k which is far below their expectations, but they r willing to let go of the house if we r willing to give them at least 30k cov in which i disagree as we have already agreed on selling at valuation price. they then asked for another month for them to do another valuation.

I waited till only a week ago then i receive news that they r now engaging an agent to deal for them as they r unable to obtain a higher valuation price for their houce n is looking at obtaining cov instead.

What r my optiuons here,
1)what can i do now, can i just insert any dates on the otp and the valuation price as i have the otp with me?
2)or should i get some legal advice and can i proceed with legal proceedings?
3)or should i go to hdb and get some consultation with them?
4)or is there any other way cause i have forgo my selection for bto as i thought i have finally gotten a resale flat?

Please help thank you.