How can I implement this logic in SQL server

You should avoid using a cursor for this - remember SQL Server is set based.

Your instructions don't really make much sense - for example you would not use currency exchange values just on negative values. I think what you mean is that you want the amounts in USD.

This also smacks a little of homework so what follows is not a complete solution - besides my initial comment above applies.

I used the following test data

declare @invoice_dump table (InvoiceNo int, currency varchar(4), Amount decimal(15,2))
declare @currency_exchange_value table (FromCurrency varchar(4), ToCurrency varchar(4), ExchangeValue decimal(15,4))
insert into @invoice_dump (InvoiceNo, currency, Amount) values
(5257,'USD',9450.00),(5257,'INR',-35212.32)
insert into @currency_exchange_value (FromCurrency,ToCurrency,ExchangeValue) values
('INR','USD',0.2666)

Note I couldn't really establish what you meant in your comments about duplicates so I have not catered for them explicitly.

This sql snippet

SELECT *, ABS(Amount) as posValue	-- Note 1
FROM @invoice_dump I
LEFT OUTER JOIN @currency_exchange_value C ON I.currency = C.FromCurrency	-- Note 2

Gives these results

InvoiceNo	currency	Amount	FromCurrency	ToCurrency	ExchangeValue	posValue
5257	USD	9450.00	NULL	NULL	NULL	9450.00
5257	INR	-35212.32	INR	USD	0.2666	35212.32

Note 1 - The ABS() function will give the absolute value of the column - effectively changing a negative value into a positive one
Note 2 - Example of using a join to get the exchange value.

The problem now is that we have two rows but we want the data from both... the best way to handle that is to use PIVOT e.g.

SELECT * FROM
(SELECT *, ABS(Amount) as posValue	
FROM @invoice_dump I
LEFT OUTER JOIN @currency_exchange_value C ON I.currency = C.FromCurrency
) src
PIVOT
(
	SUM(Amount) FOR currency IN (USD, INR) -- Note 3
)pvt

Note 3 - Notice that the currency is now a column name
That gives the result

5257	NULL	NULL	NULL	9450.00	9450.00	NULL
5257	INR	USD	0.2666	35212.32	NULL	-35212.32

so I obviously haven't got that quite right ... I need a single row per invoice so I am going to use GROUP BY

SELECT InvoiceNo, MAX(FromCurrency) as FromCurrency, MAX(ToCurrency) as ToCurrency, 
		MAX(ExchangeValue) as ExchangeValue, MAX(USD) as USD, MAX(INR) as INR 
FROM
(SELECT *, ABS(Amount) as posValue	
FROM @invoice_dump I
LEFT OUTER JOIN @currency_exchange_value C ON I.currency = C.FromCurrency
) src
PIVOT
(
	SUM(Amount) FOR currency IN (USD, INR)
)pvt
GROUP BY InvoiceN

Which gives the results

InvoiceNo	FromCurrency	ToCurrency	ExchangeValue	USD	INR
5257	INR	USD	0.2666	9450.00	-35212.32

Now that you have all the results in a single row you could add those to a temporary table or use a Common Table Expression or even plug in your simple equation.

 FromCurrency	ToCurrency	exchangevalue
INR 	ADP 	 2.094240000 
INR 	AED 	 0.051212000 
INR 	ARS 	 0.764080000 
INR 	AUD 	 0.020575000 
INR 	BDT 	 1.178190000 

ON I.currency = C.FromCurrency AND C.ToCurrency = 'USD'