When the company was paying sale consideration in instalments and 30% payment is still pending, it renders that the sale deed has not been executed. In absence of sale deed the ownership has not been transferred to the company.
You are still the owner of that land because the buyer has not paid the entire sale consideration. Your legal status (owner of the land) does not change however, you have accepted a partial amount of sale consideration.
The company is not traceable and also not paying instalments. Rate of property is going up day by day. So, in this situation you should initiate proper legal proceedings against the company because the company has breached the agreement.
The deadline for complete payment has been exhausted because the company was absent on the date fixed for the registration or execution of sale deed. Therefore, there is no need to give extra time to the company for the compliance of agreement.
You should send a legal notice to the known address of the company and inform him that due to breach of agreement on your part, the sale agreement is cancelled and the entire sale proceeds have been forfeited.
Thereafter you are free to sell the land to any other buyer. If the company responds then you are entitled to sell the land at the current market rate. The rate mentioned in the sale agreement will not apply because that agreement is not enforceable by the company. After breach of the agreement the company cannot enforce it even through the court.